“Avenger” by Frederick Forsyth

Attorney Calvin Dexter hangs his shingle in a quiet New Jersey town, has a reasonably successful practice, and takes the hills strong while triathalon training. But Dexter is no ordinary man.
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The summer before he goes to college, Ricky Colenso travels to Bosnia to volunteer as an aid worker. A few weeks later, he disappears and is never heard from again. A family grieves and is offered little hope—in the fog of that horrible time and place, the killer, too, has vanished.
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Or so it would seem. For in a world that has forgotten right and wrong, there are few like Cal Dexter who can settle the score. And so, years later, a worldwide chase is on and Dexter begins to draw a net around the killer. But this time CIA agent Paul Devereux must find a way to stop Dexter before his quest for vengeance throws the world into chaos.
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A heart-stopping novel of murder and mystery, double-cross and triple-cross, old loyalties and new hatreds, "Avenger "has all of Frederick Forsyth’s page-turning trademarks.